The present invention relates generally to devices and methods for determining analyte levels, and, more particularly, to implantable devices and methods for monitoring glucose levels in a biological fluid.
The continuous measurement of substances in biological fluids is of interest in the control and study of metabolic disorders. Electrode systems have been developed for this purpose whereby an enzyme-catalyzed reaction is monitored (e.g., by the changing concentrations of reactants or products) by an electrochemical sensor. In such electrode systems, the electrochemical sensor comprises an electrode with potentiometric or amperometric function in close contact with a thin layer containing an enzyme in dissolved or insoluble form. Generally, a semipermeable membrane separates the thin layer of the electrode containing the enzyme from the sample of biological fluid that includes the substance to be measured.
Electrode systems that include enzymes have been used to convert amperometrically inactive substances into reaction products which are amperometrically active. For example, in the analysis of blood for glucose content, glucose (which is relatively inactive amperometrically) may be catalytically converted by the enzyme glucose oxidase in the presence of oxygen and water to gluconic acid and hydrogen peroxide. Tracking the concentration of glucose is possible since for every glucose molecule converted a proportional change in either oxygen or hydrogen peroxide sensor current will occur [U.S. Pat. Nos. 4,757,022 and 4,994,167 to Shults et al., both of which are hereby incorporated by reference]. Hydrogen peroxide is anodically active and produces a current which is proportional to the concentration of hydrogen peroxide, which is directly related to the concentration of glucose in the sample. [Updike et al., Diabetes Care, 11:801-807 (1988)].
Despite recent advances in the field of implantable glucose monitoring devices, presently used devices are unable to provide data safely and reliably for long periods of time (e.g., months or years) [See, e.g., Moatti-Sirat et al., Diabetologia 35:224-30 (1992)]. For example, Armour et al., Diabetes 39:1519-26 (1990), describes a miniaturized sensor that is placed intravascularly, thereby allowing the tip of the sensor to be in continuous contact with the blood. Unfortunately, probes that are placed directly into the vasculature put the recipient at risk for thrombophlebosis, thromboembolism, and thrombophlebitis.
Currently available glucose monitoring devices that may be implanted in tissue (e.g., subcutaneously) are also associated with several shortcomings. For example, there is no dependable flow of blood to deliver sample to the tip of the probe of the implanted device. Similarly, in order to be effective, the probe must consume some oxygen and glucose, but not enough to perturb the available glucose which it is intended to measure; subcutaneously implanted probes often reside in a relatively stagnant environment in which oxygen or glucose depletion zones around the probe tip may result in erroneously low measured glucose levels. Finally, the probe may be subject to xe2x80x9cmotion artifactxe2x80x9d because the device is not adequately secured to the tissue, thus contributing to unreliable results. Partly because of these limitations, it has previously been difficult to obtain accurate information regarding the changes in the amounts of analytes (e.g., whether blood glucose levels are increasing or decreasing); this information is often extremely important, for example, in ascertaining whether immediate corrective action is needed in the treatment of diabetic patients.
There is a need for a device that accurately and continuously determines the presence and the amounts of a particular analyte, such as glucose, in biological fluids. The device should be easy to use, be capable of accurate measurement of the analyte over long periods of time, and should not readily be susceptible to motion artifact.
The present invention relates generally to devices and methods for determining analyte levels, and, more particularly, to implantable devices and methods for monitoring glucose levels in a biological fluid.
The devices and methods of the present invention allow for the implantation of analyte-monitoring devices such as glucose monitoring devices that result in a dependable flow of blood to deliver sample to the implanted device at a concentration representative of that in the vasculature. Moreover, the devices of the present invention become secured within the tissue of the subject, thereby greatly reducing or eliminating the phenomenon of xe2x80x9cmotion artifactxe2x80x9d. In addition, the devices of the present invention utilize materials that eliminate or significantly delay environmental stress cracking at the sensor interface, resulting in the ability to obtain accurate, long-term data.
These effects result, in part, from the use of materials that enhance the formation of a foreign body capsule (FBC). Previously, FBC formation has been viewed as being adverse to sensor function, and researchers have attempted to minimize FBC formation (see, e.g., U.S. Pat. No. 5,380,536 to Hubbell et al.). However, the methods and devices of the present invention utilize specific materials and microarchitecture that elicit a type of FBC that does not hamper the generation of reliable data for long periods. The devices of the present invention are capable of accurate operation in the approximately 37xc2x0 C., low pO2, environment characteristic of living tissue for extended lengths of time (e.g., months to years).
The electrode-membrane region of the devices of the present invention comprises a unique microarchitectural arrangement. In preferred embodiments, the electrode surfaces are in contact with (or operably connected with) a thin electrolyte phase, which in turn is covered by an enzyme membrane that contains an enzyme, e.g., glucose oxidase, and a polymer system. A bioprotective membrane covers this enzyme membrane system and serves, in part, to protect the sensor from external forces and factors that may result in environmental stress cracking. Finally, an angiogenic layer is placed over the bioprotective membrane and serves to promote vascularization in the sensor interface region. It is to be understood that other configurations (e.g., variations of that described above) are contemplated by the present invention and are within the scope thereof.
The present invention contemplates a biological fluid measuring device, comprising a) a housing comprising electronic circuit means and at least two electrodes operably connected to the electronic circuit means; and b) a sensor means operably connected to the electrodes of the housing, the sensor means comprising i) a bioprotective membrane, and ii) an angiogenic layer, the angiogenic layer positioned more distal to the housing than the bioprotective membrane. In particular embodiments, the bioprotective membrane is substantially impermeable to macrophages. In some embodiments, the bioprotective membrane comprises pores having diameters ranging from about 0.1 micron to about 1.0 micron. In certain embodiments, the bioprotective membrane comprises polytetrafluoroethylene, and in particular embodiments, the angiogenic layer also comprises polytetrafluoroethylene.
Particular embodiments of the biological fluid measuring device further comprise c) means for securing the device to biological tissue, the securing means associated with the housing. In some embodiments, the securing means comprises a polyester velour jacket. In preferred embodiments, the securing means covers the top surface (e.g., the top member or the top member sheath, as described further below) and a portion of the sensor interface; it should be noted that the securing means generally should not cover the entire sensor interface, as this would interfere with the ability of blood vessels to deliver sample to the biological fluid measuring device. In preferred embodiments, the securing means comprises poly(ethylene terephthalate).
In further embodiments, the sensor means of the biological fluid measuring device further comprises means for determining the amount of glucose in a biological sample. In some embodiments, the glucose determining means comprises a membrane containing glucose oxidase, the glucose oxidase-containing membrane positioned more proximal to the housing than the bioprotective membrane. In additional embodiments, the housing further comprises means for transmitting data to a location external to the device (e.g., a radiotelemetry device).
The present invention also contemplates a device for measuring glucose in a biological fluid, comprising a) a housing comprising electronic circuit means and at least one electrode operably connected to the electronic circuit means; and b) a sensor means operably connected to the electrode of the housing, the sensor means comprising i) means for determining the amount of glucose in a biological sample, the glucose determining means operably associated with the electrode, ii) a bioprotective membrane, the bioprotective membrane positioned more distal to the housing than the glucose determining means and substantially impermeable to macrophages, and iii) an angiogenic layer, the angiogenic layer positioned more distal to the housing than the bioprotective membrane.
In particular embodiments, the glucose determining means comprises a membrane containing glucose oxidase. In some embodiments, the angiogenic layer comprises polytetrafluoroethylene.
In some embodiments, the pores of the bioprotective membrane have diameters ranging from about 0.1 micron to about 1.0 micron, while in other embodiments the pores have diameters ranging from about 0.2 micron to about 0.5 micron. In certain embodiments, the bioprotective membrane comprises polytetrafluoroethylene.
Still other embodiments further comprise c) means for securing the device to biological tissue, the securing means associated with the housing. In particular embodiments, the securing means comprises poly(ethylene terephthalate). Additional embodiments comprise means for transmitting data to a location external to the device; in some embodiments, the data transmitting means comprises a radiotelemetric device.
The present invention also contemplates a method for monitoring glucose levels, comprising a) providing i) a host, and ii) a device comprising a housing and means for determining the amount of glucose in a biological fluid; and b) implanting the device in the host under conditions such that the device measures the glucose accurately for a period exceeding 90 days. In some embodiments, the device measures glucose accurately for a period exceeding 150 days, while in other embodiments, the device measures glucose accurately for a period exceeding 360 days.
The present invention also contemplates a method of measuring glucose in a biological fluid, comprising a) providing i) a host, and ii) a device comprising a housing and means for determining the amount of glucose in a biological fluid, the glucose determining means capable of accurate continuous glucose sensing; and b) implanting the device in the host under conditions such that the continuous glucose sensing begins between approximately day 2 and approximately day 25. In some embodiments, the continuous glucose sensing begins between approximately day 3 and approximately day 21. In particular embodiments, the implanting is subcutaneous.
The devices of the present invention allow continuous information regarding, for example, glucose levels. Such continuous information enables the determination of trends in glucose levels, which can be extremely important in the management of diabetic patients.
In order to facilitate an understanding of the present invention, a number of terms are defined below.
The term xe2x80x9caccuratelyxe2x80x9d means, for example, 95% of measured values within 25% of the actual value as determined by analysis of blood plasma, preferably within 15% of the actual value, and most preferably within 5% of the actual value. It is understood that like any analytical device, calibration, calibration check and recalibration are required for the most accurate operation of the device.
The term xe2x80x9canalytexe2x80x9d refers to a substance or chemical constituent in a biological fluid (e.g., blood or urine) that can be analyzed. A preferred analyte for measurement by the devices and methods of the present invention is glucose.
The terms xe2x80x9csensor interface,xe2x80x9d xe2x80x9csensor means,xe2x80x9d and the like refer to the region of a monitoring device responsible for the detection of a particular analyte. For example, in some embodiments of a glucose monitoring device, the sensor interface refers to that region wherein a biological sample (e.g., blood or interstitial fluid) or a portion thereof contacts (directly or after passage through one or more membranes or layers) an enzyme (e.g., glucose oxidase); the reaction of the biological sample (or portion thereof) results in the formation of reaction products that allow a determination of the glucose level in the biological sample. In preferred embodiments of the present invention, the sensor means comprises an angiogenic layer, a bioprotective layer, an enzyme layer, and an electrolyte phase (i.e., a free-flowing liquid phase comprising an electrolyte-containing fluid [described further below]). In some preferred embodiments, the sensor interface protrudes beyond the plane of the housing.
The terms xe2x80x9coperably connected,xe2x80x9d xe2x80x9coperably linked,xe2x80x9d and the like refer to one or more components being linked to another component(s) in a manner that allows transmission of, e.g., signals between the components. For example, one or more electrodes may be used to detect the amount of analyte in a sample and convert that information into a signal; the signal may then be transmitted to electronic circuit means (i.e., the electrode is xe2x80x9coperably linkedxe2x80x9d to the electronic circuit means), which may convert the signal into a numerical value in the form of known standard values.
The term xe2x80x9celectronic circuit meansxe2x80x9d refers to the electronic circuitry components of a biological fluid measuring device required to process information obtained by a sensor means regarding a particular analyte in a biological fluid, thereby providing data regarding the amount of that analyte in the fluid. U.S. Pat. No. 4,757,022 to Shults et al., previously incorporated by reference, describes suitable electronic circuit means (see, e.g., FIG. 7); of course, the present invention is not limited to use with the electronic circuit means described therein. A variety of circuits are contemplated, including but not limited to those circuits described in U.S. Pat. Nos. 5,497,772 and 4,787,398, hereby incorporated by reference.
The terms xe2x80x9cangiogenic layer,xe2x80x9d xe2x80x9cangiogenic membrane,xe2x80x9d and the like refer to a region, membrane, etc. of a biological fluid measuring device that promotes and maintains the development of blood vessels microcirculation around the sensor region of the device. As described in detail below, the angiogenic layer of the devices of the present invention may be constructed of membrane materials alone or in combination such as polytetrafluoroethylene, hydrophilic polyvinylidene fluoride, mixed cellulose esters, polyvinyl chloride, and other polymers including, but not limited to, polypropylene, polysulphone, and polymethacrylate.
The phrase xe2x80x9cpositioned more distalxe2x80x9d refers to the spatial relationship between various elements in comparison to a particular point of reference. For example, some embodiments of a biological fluid measuring device comprise both a bioprotective membrane and an angiogenic layer/membrane. If the housing of the biological fluid measuring device is deemed to be the point of reference and the angiogenic layer is positioned more distal to the housing than the bioprotective layer, then the bioprotective layer is closer to the housing than the angiogenic layer.
The terms xe2x80x9cbioprotective membrane,xe2x80x9d xe2x80x9cbioprotective layer,xe2x80x9d and the like refer to a semipermeable membrane comprised of protective biomaterials of a few microns thickness or more which are permeable to oxygen and glucose and are placed over the tip of the sensor to keep the white blood cells (e.g., tissue macrophages) from gaining proximity to and then damaging the enzyme membrane. In some embodiments, the bioprotective membrane has pores (typically from approximately 0.1 to approximately 1.0 micron). In preferred embodiments, a bioprotective membrane comprises polytetrafluoroethylene and contains pores of approximately 0.4 microns in diameter. Pore size is defined as the pore size provided by the manufacturer or supplier.
The phrase xe2x80x9csubstantially impermeable to macrophagesxe2x80x9d means that few, if any, macrophages are able to cross a barrier (e.g., the bioprotective membrane). In preferred embodiments, fewer than 1% of the macrophages that come in contact with the bioprotective membrane are able to cross.
The phrase xe2x80x9cmeans for securing said device to biological tissuexe2x80x9d refers to materials suitable for attaching the devices of the present invention to, e.g., the fibrous tissue of a foreign body capsule. Suitable materials include, but are not limited to, poly(ethylene terephthalate). In preferred embodiments, the top of the housing is covered with the materials in the form of surgical grade fabrics; more preferred embodiments also contain material in the sensor interface region (see FIG. 1B).
The phrase xe2x80x9cmeans for determining the amount of glucose in a biological samplexe2x80x9d refers broadly to any mechanism (e.g., enzymatic or non-enzymatic) by which glucose can be quantitated. For example, some embodiments of the present invention utilize a membrane that contains glucose oxidase that catalyzes the conversion of glucose to gluconate: Glucose+O2xe2x86x92Gluconate+H2O2. Because for each glucose molecule converted to gluconate, there is a proportional change in the co-reactant O2 and the product H2O2, one can monitor the current change in either the co-reactant or the product to determine glucose concentration.
The phrase xe2x80x9cmeans for transmitting data to a location external to said devicexe2x80x9d refers broadly to any mechanism by which data collected by a biological fluid measuring device implanted within a subject may be transferred to a location external to the subject. In preferred embodiments of the present invention, radiotelemetry is used to provide data regarding blood glucose levels, trends, and the like. The terms xe2x80x9cradiotelemetry,xe2x80x9d xe2x80x9cradiotelemetric device,xe2x80x9d and the like refer to the transmission by radio waves of the data recorded by the implanted device to an ex vivo recording station (e.g., a computer), where the data is recorded and, if desired, further processed (see, e.g., U.S. Pat. Nos. 5,321,414 and 4,823,808, hereby incorporated by reference; PCT Pat. Publication WO 9422367).
The term xe2x80x9chostxe2x80x9d refers to both humans and animals.
The phrase xe2x80x9ccontinuous glucose sensingxe2x80x9d refers to the period in which monitoring of plasma glucose concentration is continuously carried out. More specifically, at the beginning of the period in which continuous glucose sensing is effected, the background sensor output noise disappears, and the sensor output stabilizes (e.g., over several days) to a long-term level reflecting adequate microcirculatory delivery of glucose and oxygen to the tip of the sensor (see FIG. 2). Though an understanding of this effect is not required in order to practice the present invention, it is believed to be due to adequately vascularized foreign body capsule tissue in consistent contact with the sensor interface of the blood glucose monitoring device. Failure of adequate vascularization or consistent contact of tissue with sensor will result in failure of continuous glucose sensing.